A plasma process is often performed under a vacuum atmosphere by vacuuming gas from a processing chamber. In such case, a workpiece is placed on a an electrostatic chuck (ESC) that is arranged on a mounting table within the processing chamber.
The electrostatic chuck includes a chuck electrode made of a conductive sheet and dielectric members arranged at the front surface and backside surface of the chuck electrode. In the case of performing a plasma process, a workpiece such as a wafer is electrostatically attracted to the electrostatic chuck by a Coulomb force that is generated when a voltage from a DC voltage supply is applied to the electrostatic chuck, and the plasma process is performed in such a state. Also, a heat transfer gas is supplied between a backside surface of the wafer and the front surface of the electrostatic chuck. In the case of de-chucking the workpiece from the electrostatic chuck after the plasma process is ended and the voltage applied to the electrostatic chuck is turned off, inert gas is introduced into the processing chamber and the pressure within the processing chamber is maintained at a predetermined pressure. In this state, an opposite polarity voltage of the voltage applied to the electrostatic chuck during the plasma process is applied to the electrostatic chuck, and the opposite polarity voltage is turned off thereafter. In this way, a discharge process is performed for discharging (removing) electric charges in the electrostatic chuck and the workpiece. Then, support pins are raised to lift the workpiece from the electrostatic chuck and de-chuck the workpiece from the electrostatic chuck.
However, the front surface and the backside surface of the electrostatic chuck change over time. For example, substances such as reaction products generated during a plasma process adhere to and gradually accumulate on the front surface of the electrostatic chuck to form an insulating film. The accumulated substances are easily charged and tend to retain the electric charge. Accordingly, the electric potential of the front surface of the electrostatic chuck may change over time. In turn, the attraction force of the electrostatic chuck may change as a result of the accumulated substances. That is, an electric charge may be accumulated within the insulating film formed on the front surface of the electrostatic chuck, and a residual charge may remain on the surface layer of the electrostatic chuck even when the voltage applied to the electrostatic chuck is turned off. The residual charge may not be removed even when the above discharge process is performed. Thus, an electrostatic attraction force from the residual charge may remain when the support pins are lifted, and as a result, the workpiece may be damaged and/or normal loading operations of the workpiece may be hampered, for example.
In this respect, Patent Document 1 discloses a technique for pre-emptively avoiding a situation in which a workpiece such as a wafer is prevented from being de-chucked from the electrostatic chuck owing to such a residual charge. The technique involves detecting the state of the residual charge attracting the wafer upon turning off the voltage applied to the electrostatic chuck from the DC voltage supply and lifting the wafer from the electrostatic chuck, and determining a replacement timing for replacing the electrostatic chuck based on the detection result. In Patent Document 1, the state of the residual charge on the electrostatic chuck is detected based on the torque or rotation speed of a drive motor for lifting the support pins supporting the wafer.